Nick McGuffin, who was Leah's boyfriend at the time of her death in 2000, is expected to stand trial for her murder on May 10, 2011.

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Missing Teen's Friend Regrets Fighting

ABC News' "20/20" was given exclusive access during the re-opening of this decade-old cold case with a fresh investigative team, including on the day police went in to apprehend McGuffin, the person they had suspected from the beginning.

After a grand jury's indictment was handed down, police arrested McGuffin on Aug. 24, 2010 and charged him with murder.

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Cold Case: What Happened to Oregon Teen?

When police arrived at McGuffin's home, he was retrieving his mail and still wearing a uniform from his job as a short-order cook. He stood quietly as police read him his rights and handcuffed him.

"I didn't do it. Love of my life, man," he told "20/20" at the time of his arrest. "They have nothing else to go on, and I'm the boyfriend."

McGuffin pleaded not guilty and continues to maintain his innocence. His bail was set at $2 million.

Leah was a high school freshman when she disappeared in August 2000 after leaving a friend's house. Police initially treated the disappearance as a runaway teen case. Weeks later, the girl's decomposing body was found on a wooded roadside slope a few miles outside town.

Authorities said circumstantial evidence initially pointed to then-18-year-old McGuffin. But there was no hard evidence to link him to the crime.

Leah's best friend, Cherie Mitchell, said she thought Leah's relationship with the high school senior was bad for her.

"I think that he was kind of dangerous and, you know, maybe she liked the danger side of him," Mitchell said.

Leah's mother, Cory Courtwright, said that she did not approve of her daughter's relationship either.

"I found out that they were being sexually active, and that was disturbing for me," Courtwright said.

On the night of Leah's disappearance, Mitchell confronted her friend and told Leah she should break-up with McGuffin. Furious, Leah stormed out. She last was seen walking alone near a gas station in downtown Coquille, but she never made it home.

Justice for Leah: A Small Town Is in Uproar Over the Teen's Death

Former Police Chief Mike Reeves, who was head of the town's police department in 2000, turned down several requests to be interviewed, but recently told ABC News' Jim Avila that five days passed after Leah was reported missing before he was convinced of foul play.

Two blood-splattered gym shoes found miles apart had surfaced during that time and it would take investigators another month to find her body, but still no arrests were made.

As the years dragged on, the small community of Coquille, a town of about 4,000 residents, became more furious that Leah's murderer hadn't been caught.

A new police chief, Mark Dannels, vowed to solve the case and a team of forensic experts known as the Vidocq Society, based in Philadelphia, was brought in to advise on the case.

A crime-solving team made up of former FBI special agents, police officers, forensics experts and profilers, the Vidocq Society specializes in solving cold cases. One of the members, forensic pathologist Richard Walters, traveled to Oregon to review the evidence early last spring.

After reexamining Leah's bloody shoes and how her body was dumped, Walters labeled Leah's killer a "power assertive crime type."

"Power assertive is that muscle flexing, tectonic kind of braggart who thinks he's John Wayne, who wants to be a bigger man than what he is," Walters said. "The end goal was to get [Leah] dead and to get her dead in the most efficient way."

With this fresh perspective on the case, police were even more convinced that their prime suspect from the beginning, Leah's boyfriend, Nick McGuffin, was her killer.

But without a DNA match or an eyewitness, they needed enough evidence to build a circumstantial case that would hold up in court. They again reviewed integration tapes of McGuffin's account of what happened the night Leah disappeared.

McGuffin claimed he went to Leah's friend Cherie Mitchell's house to look for her around 9 p.m., but Leah already had stormed off. Then, he said he drove around for hours in his 1967 Ford Mustang looking for her.

Police and Friends Begin to Suspect Leah's Boyfriend, Nick McGuffin

Police said they had several witnesses who saw McGuffin acting strangely that night, including Mitchell, who said he showed up at her house a second time at around 10 p.m.

Police also noticed that McGuffin seemed convinced that the worst had happened to Leah when they first interviewed him in 2000, and that he kept reverting back to past tense in his written statements.

"He tells us that Cherie was, was, her best friend, and the question is: How does he know that Cherie and Leah are no longer best friends?" said Mark Mclish, a handwriting expert who specializes in statement analysis. "The answer is 'cause he knows that at this point in time, Leah's dead."

Police then reexamined McGuffin's Mustang, even though it had already been sprayed with a chemical called luminal to test for the presence of blood during the initial investigation in 2000.

"There was nothing in there," said police chief Mark Dannels. "It had been cleaned. It had been wiped."

During the second round of searching, police got a huge break in the case.

Another witness, Kristen Steinoff, was questioned, and told police that McGuffin had stopped by her house around midnight on the night of Leah's disappearance. She said she and McGuffin did drugs, and when he tried to have sex with her, she refused. He also barrowed her car that night -- a Kia.

"Here's a guy that's worried about his girlfriend, that he's frantic about her ... but he's making a move on this girl," Dannels said.

The Kia had been sold, but detectives traced it back to its new owner in northern Idaho. After searching it, they found it, too, had been thoroughly cleaned, with air fresheners placed in the passenger side compartment and the trunk.

Steinoff was bought in again for questioning. She insisted she knew nothing about what happened to Leah, but then revealed to police that she had seen McGuffin twice that night. When he came around a second time, he was wearing different clothes and had threatened her.

"He started saying to keep my mouth shut," she said. "Then, I kind of figured that he did have something to do with it because of the way he was acting."

Police Arrest McGuffin After 100 People Testify at a Grand Jury Hearing

By that point, it was 2010 and police believed they had enough witnesses to prove McGuffin had lied about his whereabouts and he was the only one with a motive.

Steinoff and 100 other people testified in front of a grand jury, hoping to convince it that there was enough evidence to indict McGuffin for homicide. Shortly after the decision was handed down, police arrested him.

The question still remains: If McGuffin did kill Leah, what was his motive?

According to police, Leah allegedly had told some of her friends that her period was late and she possibly was pregnant. He was 18 and she was 15, and police suspect that McGuffin feared statutory rape charges.

After his arrest, McGuffin's family claimed that drug dealers should be investigated for Leah's death.

Leah's mother, Cory Courtright, said she doesn't believe there will ever be closure for her, but believed her daughter deserved justice.

"I just can't stand the thought of somebody getting away with taking such a beautiful, young life. It's just wrong," she said. "Whoever did this needs to pay."